In forming substrate glasses for various kinds of displays, especially metals or oxide thin films on their surfaces, the following characteristics have been required.
(1) If an alkali metal oxide is contained, since alkali metal ions are diffused into a thin film and degrade the film characteristics, alkali metal ions should not be substantially contained.
(2) In case of being exposed to a high temperature in a thin film formation process, the strain point should be high to be able to suppress the contraction (thermal contraction) due to the deformation of the glass and the structural stabilization of the glass to a minimum.
(3) Sufficient chemical durability with respect to various kinds of chemicals that are used in semiconductor formation should be provided. In particular, the durability with respect to buffered hydrofluoric acid (BHF: a mixed solution of fluoric acid and ammonium fluoride) for the etching of SiOx or SiNx, medicinal fluids containing hydrochloric acid and which are used in the etching of ITO, various kinds of acids (nitric acid, sulfuric acid, etc.) that are used in the etching of metal electrodes, and the alkali of a resist peeling-off liquid should exist.
(4) Defects (foam, striae, inclusions, pits, scratches, etc.) should not be present internally and on the surface.
In addition to the aforementioned requirements, the following states have recently been in demand.
(5) The lightness of displays is required and a glass itself is desired to have a low density.
(6) The lightness of displays is requested, and a substrate glass is desired to have a reduced thickness.
(7) In addition to amorphous silicon (a-Si) liquid crystal displays used up to now, polycrystalline silicon (p-Si) liquid crystal displays with a slightly high heat treatment temperature have been prepared (a-Si: about 350° C.→p-Si: 350-550° C.).
(8) In order to raise the productivity or heat impact resistance by raising the temperature rise and drop rate of a liquid-crystal-display preparation heat treatment, a glass with a small average thermal expansion coefficient has been in demand.
On the other hand, as the dryness of etching is advanced, the demand for BHF resistance has been weakened. In glasses used up to now, to improve the BHF resistance, glasses containing 6 to 10 mol % of B2O3 have frequently been used. However, B2O3 tends to lower the strain point. As examples of an alkali-free glass containing no B2O3 or with a low content, the following can be mentioned.
In Patent Document 1, a SiO2—Al2O3—SrO glass containing no B2O3 is described; however, since 77 mol % or more of SiO2 is contained therein, the temperature required for its melting is high and a difficulty is caused in its preparation.
In Patent Document 2, a SiO2—Al2O3—SrO crystallized glass containing no B2O3 is described; however, since 68 mol % or more of SiO2 and 18 mol % or more Al2O3 are contained therein, the temperature required for its melting is high and a difficulty is caused in its preparation.
In Patent Document 3, a glass containing 0 to 5 wt % B2O3 is described; however, since 11 mol % or more of CaO is contained therein, its devitrification temperature is high, the content of phosphorus present as an impurity in limestone used as the raw material of CaO is high, and a leak current is likely to be generated in transistors that are prepared on a glass substrate.
In Patent Document 4, a glass containing 0 to 3 wt % B2O3 is described; however, since 8 mol % or more of SrO and CaO are respectively contained therein, its average thermal expansion coefficient at 50 to 300° C. exceeds 40×10−7/° C.
In Patent Document 5, a glass containing 0 to 5 mol % B2O3 is described; however, since 15 mol % or more of SrO is contained, its average thermal expansion coefficient at 50 to 300° C. exceeds 50×10−7/° C.
In Patent Document 6, a glass containing 0 to 5 wt % B2O3 is described; however, since 12 mol % or more of BaO is contained therein, its thermal expansion is large and the density is high.
In Patent Document 7, glasses containing 0 to 8 wt % B2O3 are described and classified as “a glass containing 55 to 67 wt % SiO2 and 6 to 14 wt % Al2O3” (group a) and “a glass containing 49 to 58 wt % SiO2 and 16 to 23 wt % Al2O3” (group b). However, in group a, since the content of SiO2 is high, silica sand used as the raw material of SiO2 cannot be completely melted but remains as undissolved silica sand. In group b, since the content of Al2O3 is high, the devitrification temperature is markedly high.
In order to solve the problems with the glasses described in Patent Documents 1 to 7, an alkali-free glass described in Patent Document 8 is proposed. The alkali-free glass described in Patent Document 8 has a high strain point and can be molded by the float process, so that it is said that this glass is suitable for substrates for displays and substrates for photomasks.
However, as a method for producing high-quality p-Si TFT, although there is the solid-phase crystallization method, it is required that its strain point be higher to apply this method.
In addition, due to the request in glass preparation processes, especially melting and molding, it is in demand that the viscosity of the glass be lower and that the devitrification be low.